Thermal energy storage materials (TESMs) are known and have been used in applications for storing heat for subsequent use. Many TESMs are phase change materials, meaning they undergo a phase change, typically between solid state and liquid state, and can store (or release) a considerable amount of the heat, regarded as latent heat from the phase change. Many of these phase change materials include mixtures of compounds, such that the mixture has a lower liquidus temperature than the pure compounds or elements used in the mixture. See generally, Chapter 3, I. Dincer and M. A. Rosen, Thermal Energy Storage Systems and Applications, John Wiley & Sons, London, 2002.
Attention has been directed toward TESMs for use at temperatures below about 85° C. Much of this body of work utilizes mixtures of hydrous metal salts. For example, U.S. Pat. No. 6,627,106 discloses various phase change materials comprising ternary mixtures of magnesium nitrate hexahydrate with other metal nitrates. These mixtures having phase changes from about 52° C. to about 69° C., depending on the metal nitrates being combined and on the concentration of each metal salt. U.S. Pat. No. 5,785,884 discloses similar ternary mixtures of magnesium nitrate hexahydrate with sodium nitrate and potassium nitrate. These hydrous mixtures of metal nitrates have solid to liquid phase transitions between 60° C. and 85° C. U.S. Pat. No. 5,728,316 describes binary mixtures of magnesium nitrate hexahydrate and lithium nitrate where the molar ratio of magnesium nitrate hexahydrate to lithium nitrate is from 86:14 to 81:19 with single melting temperature in the range of 71° C. to 78° C. U.S. Pat. No. 6,083,418 discloses phase change materials comprising a mixture of two metal nitrates (an alkali metal nitrate and an alkaline earth metal nitrate) with an excess of water, such that the phase change material has a small change in density between the solid phase and the liquid phase. The water concentration ranges from 27.9% to 37.2% by weight of the phase change material, with the specific concentration range of the water dependent on the metal salts being mixed. U.S. Pat. No. 5,348,080 shows mixtures of water, sodium nitrate, and potassium nitrate and describes phase change materials having a solid to liquid transition temperature below 0° C. See also, U.S. Pat. No. 5,591,374.
Attention also has been directed toward anhydrous mixtures of metal salts as phase change materials having very high phase transition temperatures. For example, Kerslake, T. W. and M. B. Ibrahim, “Analysis of thermal energy storage material with change-of-phase volumetric effects,” Journal of Solar Engineering, 115:1, (1993) pp. 22-31, disclose anhydrous mixtures of lithium fluoride and calcium fluoride which melt at 1,040 K (767° C.). U.S. Pat. No. 4,657,067 discloses a variety of binary and ternary metal compositions which can be used as thermal energy storage materials. These mixtures all have melting or liquidus temperatures above 500° C. Other phase change materials for use in various applications are disclosed in U.S. Pat. Nos. 4,421,661 and 5,613,578. In a paper presented to The Modelica Association, entitled “Analysis of steam storage systems using Modelica” (Modelica 2006, September 4-5), Buschle, et al attempt to model steam storage systems that use unspecified eutectic mixtures of “salts such as: lithium nitrate (LiNO3), lithium chloride (LiCl), potassium nitrate (KNO3), potassium nitrite (KNO2), sodium nitrate (NaNO3), sodium nitrite (NaNO2) and calcium nitrate (Ca(NO3)2).”
Heretofore, efforts to apply TESMs in commercial applications have also been complicated by difficulties in achieving satisfactory performance in service. Though a TESM may be known to have certain attributes to qualify it as a heat storage material, the assembly of such TESMs into a functionally operative system has been complicated by the unpredictability of the materials and other considerations, such as TESM interactions in service with other system materials. For example, corrosion resistance has proven to be a complicating factor for some systems. Many materials known to be phase change materials are also corrosive in many environments. It may also prove difficult to predict how mixtures of such materials will fare in a desired application. Consider also that, typically, the TESMs must be packaged in some device that affords efficient heat exchange. Considerations such as the compatibility of TESMs with these devices potentially can significantly deteriorate the performance of a particular TESM, even though the TESM is regarded theoretically as a good performing TESM. For example, some water-containing TESMs have been observed to cause relatively rapid corrosion of device packaging. Unpredictable kinetics also have complicated the adaptation of various TESM material systems, as has been durability in the face of intensive thermal cycling.
In view of the above, there continues to be a need for new and efficient TESMs specifically for use at one or more temperatures of about 85° C. to 300° C. and exhibiting one or more of a relatively high energy storage density, a relatively high heat of fusion, relatively low corrosivity toward common materials of construction, relatively rapid crystallization kinetics, long cycle life and long calendar life, and otherwise good compatibility with components of systems in which the TESMs are used.